Mhd generator

ABSTRACT

1,111,079. Lasers. SIEMENS-SCHUCKERTWERKE A.G. 23 Sept., 1966 [25 Sept., 1965], No. 42700/66. Heading H1C. [Also in Divisions F1, F4 and H2] A pulsating gas flow is produced in an MHD generator duct 22 by igniting the fuel air mixture in combustion chamber 4 by means of a laser beam 7. Combustion occurs at constant volume with a frequency determined by the firing frequency of laser 11 and the combustion products flow through the magnetic field to generate a voltage at opposed electrodes 23-30. The combustion chamber may be joined to the generator duct by a pulsating jet tube of non- resonant length and shaped to provide good combustion, Fig. 2 (not shown), or good inlet suction. Fig. 3 (not shown). In a modification, Fig. 4 (not shown), flap valves may be used to control the inlet of the fuel/air mixture. Additional combustion chambers may be used all feeding a single generating duct and A.C. may be generated from pulsating D.C. by suitable choice of repetition frequency. The laser comprises a ruby crystal 17 which is pumped by flash lamp 18 controlled by capacitor and ignition coil 19 and the beam is focused by lens 16. It is stated that fuel oil/ air mixture of 1 : 1000 may be ignited by impulses of 0.5 millisec. duration with an energy of 1.5 watts/sec. The MHD generator system may be used as a thrust nozzle for aircraft.

B. BERBRICH MHD GENERATOR Nov. 25, 1969 2 Sheets-Sheet l Filed Sept. 22, 1966 Nov. 25, 1969 e. BERBERICH MHD GENERATOR 2 Sheets-Sheet 2 Filed Sept. 22. 1966 United States Patent U.S. Cl. 310--11 4 Claims ABSTRACT OF THE DISCLOSURE MHD generator assembly includes a channel disposable transversely to a magnetic iield, burner means for supplying a hot gaseous stream intermittently through the channel and including a constant-volume combustion chamber, means for periodically supplying ignitable fuel-oxidant mixture to the chamber, laser device for directing an intermittent beam of laser radiation pulses into the chamber to lgnite the mixture supplied thereto, means for synchronizing the frequencies respectively of the supply of mixture to the chamber and the pulses of the laser beam, and an oscillation tube located between chamber and channel and traversible by periodic succession of hot and cold gas pulses from the chamber, the tube having a nonreslonant length relative to the periodic wavelength of the pu ses.

My invention relates to magnetohydrodynamic or magnetoplasmadynamic generators, hereinafter referred to as MHD generators, which operate by a pulsating sequence of com'bustions. In the course of the pulsating combustion, a periodically initiated constant-volume combustion occurs wherein a combustible mixture present in a combustion chamber is discharged through an oscillation tube after it has been ignited and explosively consumed. With suitable dimensioning, a pressure wave is reflected back from the end of the oscillation tube and ignites a newly charged mixture in the combusion chamber. The combustion chamber and the oscillation tube form a so-called Schmidt tube. Further details thereof can be obtained, for example, from the papers in VDI-Journal (Journal of the German Engineering Society), volume 92 (1950), No. 16,` pages 393-399, and in volume 94 (1952), No. 3l (pages 1005-1008).

With such burners, the oscillation tube is traversed -by succeeding hot and cold gas zones. The frequency of the hot and cold zone sequences, which can be called zone frequencies for the sake of simplicity, is determined essentially by the dimensions of the oscillation tube and the combustion chamber. A combustible mixture can be supplied to the combustion chamber by means of automatic induction in the negative-pressure range of the discharging pressure wave or by injection. When adapted for MHD generators, the combustion chamber with its internal combustion space and the supply devices, together with the oscillation tube form the burner proper.

The advantages of the pulsating combustion are that in the hot zone essentially higher temperatures are achieved than have heretofore lbeen possible with the same fuels in a constant-pressure combustion process. This is due to the fact that with a pulsating combustion it is possible to effect a virtually complete combustion with a stoichiometric distribution of the starting components of the working gas.

Various difficulties have, however, arisen in the course of employing the pulsating combustion for MHD generators. Thus, all attempts to maintain the zone frequency adequately constant or steady for prolonged operating periods without individual tuning of the respective burners have proved unsuccessful heretofore. The kinetic processes ICC of the reaction are namely still largely unknown so that the ignition sequence cannot be properly controlled as yet. Such burners can therefore not be installed at present in industrial plants. A further diiculty is encountered in the operation of an MHD generator with segmented electrodes for the production of direct current. In such MHD generators, opposing electrode pairs are located in a hot gas beam having the properties of a plasma, electric power being drawn from the electrode pairs when a magnetic field is applied perpendicularly to the ow direction of the gas beam and to an imaginary connecting line between the electrodes of each pair of electrodes. With a working gas consisting of hot and cold zones, only direct current can then be drawn off when the zone frequency is high. In practice, the Zone frequency should be greater than 400 cycles per second (c.p.s.).

The desirability of employing pulsating combustion for MHD generators is due to the effective increase therewith of the generator power output with the conductivity and temperature of the working gas. The average temperature throughout the hot and cold zones determines the loading on the MHD channel wall, that average temperature being lower than the temperature of the hot zones which is the determining factor for the power output.

Since the zone frequency is inversely proportional t0 the duration time of the pressure waves in the oscillation tube, the latter must be relatively short so as to permit high zone frequencies. On the other hand, a linear wave propagation is produced only when the oscillation tube diameter is small with respect to the length of the tube. The disadvantage for MHD generators thus follows directly from the fact that the throughput is relatively small. It has therefore been proposed to separate the functions defined by the zone frequency and the throughput quantity. The suggestion envisions two stages whereby the oscillation tube of the rst stage denes the zone frequency and the detuned second stage serves solely for increasing the throughput, whereby at most a third stage is necessary to again attain the desired temperatures of the working gas.

It is accordingly an object of my invention to provide an MHD generator which avoids the aforementioned disadvantages of the known MHD generators and more particularly to provide an MHD generator with pulsating combustion so that the advantages of periodic constant-volume combustion are made available and the characteristics thereof also remain substantially constant even when the generators are mass-produced.

With the foregoing and other objects in view, I provide in accordance with my invention an MHD generator wherein a very tight beam periodically released from an excitation energy source which passes through an optical transmitter with selective fluorescent medium (laser beam) serves for igniting a combustible mixture supplied periodically to the combustion chamber of the MHD generator.

Recognition is accorded the fact that a complete combustion in starting components supplied in stoichiometric ratio is possible, when with the turbulence present due to pressure and relative motion between fuel particles and oxidant, the -oxidation marginal layers of the fuel are torn off. It has been found that this is also possible for a combustion Chamber for constant-volume combustion without any reflected wave.

It has also been previously suggested to ignite a diesel oil mist by laser beam when tiring a furnace or heating vessel. However, this suggested method relates to a constant-pressure combustion and not a constant-volume cornbustion as in the instant application. Thus, in accordance with a further feature of my invention, I arn able to accommodate the zone frequency to the operating requirements without any changes in the construction at the burner or at the generator channel but rather by adjusting the pulse frequency of the laser.

Other features which are considered as characteristic for the invention are set forth in the appended claims.

Although the invention is illustrated and described herein as embodied in MHD generator, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, in which:

FIG. 1 is a schematic anl longitudinal sectional view of an MHD generator constructed in accordance with my invention;

FIGS. 2 to 4 are longitudinal views partly in section and partly broken away showing different embodiments of the MHD generator of FIG. 1; and

FIG. 5 is a schematic circuit diagram for the MHD generator of my invention.

Referring now to the drawings and first particularly to FIG. l thereof, there is shown in longitudinal section an MHD generator having a burner 1 connected to the generator channel 2. The burner 1 comprises a combustion chamber 3 with a pear-shaped combustion space 4 into which nozzles 12 extend from a supply source 5 for the components of a working gas. In addition, a laser ignition device 11 is built onto the combustion chamber 1. An opening 6 is provided in the wall of the chamber 3 to permit the laser beam 7 to enter the chamber 3.

The combustion space 4 and an outlet opening 8 therefor from the chamber 3 are of such dimensions as to achieve constant-volume combustions when the throughput from the supply S is so adjusted that the ignited working gas respectively has substantially flowed out of the outlet 8 before new working gas components for the next combustion are supplied to the combustion space 4. Otherwise there would be a continuous flow through the outlet 8. The laser beam 7 can be focused on the center line 9 passing through the -outlet 8 so that the focal point is located approximately between the center of the combustion space 4 and the outlet 8. Thus, the combustion is largely propagated into the combustion chamber in the same space. Particularly high temperatures are thus achieved. If ignition occurs more in the center of the combustion space 4, particularly for spherical combustion chambers, more rapid combustion sequences and higher zone frequencies can be obtained.

'Ihe supply 5 can in general include the supply nozzle 12 for fuel and seed material mixture, as well as the nozzle 13 for atomized or finely divided fuel and the nozzle 14 for additional combustion air.

The following operations take place during a pulsating combustion in the combustion chamber 3.

The starting components for a combustion are added premixed, for example by injection. Macromixing then occurs at the combustion front whereby the fuel liquid is comminuted into cells which are, however, still larger than molecules. In the shock wave, the fuel cells are then continuously entrained in the gasified oxidation skins or lamina in the vortex thereof whereby a further combustion and intermixing occurs. This phase constitutes micromixing, followed by molecular diffusion thereafter. The vortex in the shock wave is thus produced due to the relative motion between the larger masses of sloW moving fuel particles and more rapidly moving particles of the oxidation medium.

The ignition device 11 substantially comprises a laser head 15 and a lens 16. The laser head contains, for example, a ruby crystal 17 and a flash lamp 18 as excitation energy source. The ignition device for the flash lamp is shown schematically by the box 20 and is illustrated in greater detail in FIG. 5 and described more fully hereinafter. The diaphragm or shutter of the laser is shown in the protective tube 21. The focal distance distance of the lens 16 can be one meter, as for example in the embodiment of FIG. 1. The lens 16 is mounted in the protective tube 21 so that the laser beam 7 is focused on the charge in the combustion chamber space 4. Since the opening angle of the ruby crystal employed is about 30 minutes, the diameter of the beam in the region of highest energy density, i.e., at the focal point 10 in the instant case, is approximately 2 millimeters. The region of highest energy density then extends over several centimeters in length. With the described ignition device 11, heating oil comminuted and heated to approximately C. and having a mixing ratio of oil to air of 1:1000, can be ignited by a pulse with an energy density of 1.5 watts per second with a pulse duration of 0.5 millisecond. Light pulses of 0.3 to 2 milliseconds are thus suitable for exciting the laser crystal. The pulse sequence of the laser beam is thus controlled by adjusting the ignition device 20 for the fiash lamp 18.

The generator channel 2 is connected to the outlet 8 of the combustion chamber 3. The diameter of the channel 2 can be larger than the diameter of the outlet 8 and can be made as large as desired so that the flow space 22 of the generator channel does not act as an oscillation tube,

Formation of a refiecting wave can also be prevented with a narrow generator channel if damping members are provided at the end of the channel. Such damping members can be a diffuser or the reservoir of a heat exchanger. The MHD generator is then operated with supersonic fiow. Otherwise a supersonic nozzle can also serve as a barrier or gate for the reflected waves.

It is a particular advantage that the MHD generator constructed in accordance with my invention can be placed in supersonic operation easily by employing pulsating combustion. If MHD generators, namely with burners which are to be ignited with the reflecting wave, are to operate supersonically, intermediate holders have to be connected between the oscillation tube and the speedincreasing Laval supersonic nozzle for uncoupling purposes, which cause disturbances and losses.

Segmented electrodes can be located in the generator channel so that the respective oppositely disposed electrodes 23 and 24, 25 and 26, 27 and 28, as well as 29 and 30 form electrode pairs from which the leads 31 can draw electric power respectively. Of course, as is well known for MHD generators, a magnetic field is applied perpendicularly to the ow direction of the working gas and perpendicularly to an imaginary connecting line respectively between the electrode pairs. This magnetic field can be produced by exciter windings located above and below the plane of the drawing of FIG. l, one of the excitation coils being indicated at 32.

The electrodes 23 to 30 can be embedded in electrically insulating channel walls. For this purpose, the channel walls can be constructed of ceramic-like materials or of cooled metal blocks suitably insulated from one another by spaces which are filled with heat-resistant electrically insulating cement. The channel walls 33 and 34 shown in FIG. 1 in cross section are closed by additional enclosing channel walls located above and below the plane of the drawing in FIG. l.

A tube 35, acting as an oscillation tube, as shown in FIG. 2, can be connected between the combustion chamber 3 and the generator channel 2. If the length of the tube 35 is not adjusted to the zone frequency, i.e., is above the resonance frequency, no resonance occurs. The tube 35 then acts only as a weak oscillation tube, whereby the ignition frequency and the zone frequency therewith are determined by the pulse sequence of the ignition device. Such a detuned oscillation tube can form a negative pressure, however, in the combustion space 4 after termination of an explosive combustion, which is ade- 5 quate for inducting a new charge of starting component materials. Combustion chamber 3 with the supply 5 and ignition device 11 on the one hand and the detuned oscillation tube 35 then form a burner which furnishes the generator channel 2 with working gas.

The known effects of the shape of the combustion space and the form of the oscillation tube on the operating characteristics thereof can also be utilized for an MHD generator constructed in 'accordance with the invention. Thus, a structural shape such as that of FIG. 2 affords an especially good combustion, whereas the structural shape of FIG. 3 develops an especially strong suction or negative pressure after each of the constant-volume combustions.

The supply 5 of the combustion components can be connected with the combustion space 4 by channels 36 (FIG. 4) which open into the combustion space 4 and are closable by ap or suction valves 37.

FIG. 4 shows the valves 37 in open condition at the instant of induction following a constant-volume combustion. The arrows 38 indicate the ow direction of the inducted components. FIG. 4 further shows a hot zone which, at the instant depicted, is discharging through the outlet 8 in the direction of the arrow 39. The valves 37 are closed when a new combustion is ignited, in almanner described more fully hereinafter with regard to FIG. 5.

In FIG. 5 there is shown a schematic circuit diagram containing the components for igniting the flash lamp 18 as well as for controlling the fuel supply of the MHD generator. A control mechanism 61 synchronizes the ignition sequence of the ash lamp 18 with the fuel feed. Also shown is a nozzle control deviceV 5 for actuating the valves 37 (FIG. 4) of the fuel supply channels 36 in the combustion space 4. The ignition device 20, as shown in FIG. 5, is provided with a transformer 40 which steps up the line voltage to approximately 1.5 kilovolts (kv.). A capacitor 41 and rectifier 42 are connected to the secondary winding of the transformer 40. In a narrower sense, the ignition device comprises an ignition coil 43 in which a voltage pulse in the order of magnitude of kilovolts (kv.) is supplied by a spark gap 45 from a circuit 44 which ignites the flash lamp 18. The energy for the ash lamp 18 is supplied by the capacitor 41 from the transformed line voltage. The ignition operation is initiated by a current pulse from the control device 61 to the transformer 46. The assembly device and the control device 61 are connected to the alternating current conductors 47 of a power line.

The control device 61 consists primarily of a timelimit switch 48 which, in the interest of simplicity, is shown as a mechanical member. The time-limit switch 48 has a contact arm 49 rotatable in a clockwise direction so as to sequentially engage the contacts 50 and 51. A lead 53 connects one phase of the voltage line 47 with the pivot about which the switch arm 49 rotates. Leads 54 and 55 extend respectively from the contacts 50 and 51 and connect with one terminal of the terminal pairs 62a and 62h respectively leading to the nozzle control device 5 and the ignition device 20 respectively. A lead 57 connects the other terminal of the respective terminal pairs 62a and 62b with the return voltage line 47. As long as the contact arm 49 engages a contact, the circuit r,for the nozzle control device 5 or the ignition device 20, as the case may be, is closed. The contacts 50 and 51 can be movable relative to each other, if desired, in order to adjust the time period for injecting the fuel prior to ignition of the laser.

The nozzle control device 5 can consist essentially of a magnetic valve 58. When the circuit to the device 5 is closed by the timer 48, current ows through the coil 59 of the magnetic valve 58 and electromagnetically displaces an iron core, for example, to actuate the valve member 60, which can correspond, for example, to the valve member 37 of FIG. 4. Thus, in the simplest case, the valve member 60, or a member thereof, can open the pressurized supply conduits of fuel, oxidant and seed material by excitation of the coil 59.

For good control of the ignition sequence and therewith the zone frequency with an MHD generator of my invention, a number of combustion chambers can be installed to operate in parallel. It is then possible to furnish generator channels of varying orders of magnitude with burners of very small size. With suitable generator channels and suitable zone frequencies, pulsating direct current can also be lproduced which can be converted into alternating current.

The MHD generator of my invention is particularly suited as a thrust nozzle for the propulsion of missiles and space vehicles.

I claim:

1. MHD generator assembly `comprising a generator channel disposable in a magnetic held extending transversely to the axis of `said channel, and burner means for supplying a hot gaseous stream intermittently to said generator channel for traversing said channel, said burner means comprising a combustion chamber for constant- Volume combustion, means for periodically supplying ignitable fuel-oxidant mixture into said combustion charnber, ignition means comprising a laser pulse device for directing an intermittent beam of laser radiation pulses into said combustion chamber so as to repeatedly ignite fuel-oxidant mixture supplied thereto, means for synchronizing the frequency of supplying the fuel-oxidant mixture into said combustion chamber with the pulse frequency of said laser beam, and an oscillation tube located between said combustion chamber and said generator channel, said oscillation tube being traversible by a periodic succession of hot and cold gas pulses supplied from said combustion chamber, said oscillation tube having a nonresonant length relative to the periodic Wavelength of said pulses.

2. MHD generator assembly according to claim 1, wherein said generator channel is directly connected to said combustion chamber.

3. MHD generator assembly according to claim 1, wherein said mixture supplying means is adapted to inject seed material into said combustion chamber with said fuel and oxidant.

4. MHD generator assembly according to claim 1, including valve means in said combustion chamber actuable by said `synchronizing means for admitting the fuel and oxidant to said combustion chamber.

References Cited UNITED STATES PATENTS 3,154,703 10/1964 Zahavi 310-11 3,177,651 4/ 1965 Lawrence 60-202 DAVID X. SLINEY, Primary Examiner 

